1. Field of the Invention
This invention relates to a vehicle navigation system and more particularly to map drawing for such a system.
2. Description of the Related Art
In a vehicle navigation system, map data corresponding to the present position of the vehicle is read out from a map data storage medium, such as a compact disk read-only memory (CD-ROM), in order to draw (display) a corresponding map on a display screen. As the vehicle travels, a vehicle position mark is moved on the map or the map is scrolled while the vehicle position mark is fixed at a predetermined position (e.g., the position corresponding to a center of the display screen).
The map data includes (1) road "layers" including node data link data, intersection data and the like, (2) background "layers" for displaying objects on a map, and 3) character "layers" for displaying names of cities, towns, villages and the like. A map image displayed on the display screen is generated on the basis of the background layers and the character layers. Map matching processing and guide route searching are performed on the basis of the road layers.
Maps at a plurality of scales, from a small scale for a wide-area map to a large scale for a detailed map, are stored in the CD-ROM. A map at the scale selected by the user is drawn on the display screen. In a detailed map, a small area occupies the entire display screen. In such a detailed map of course, even minor (narrow) roads are displayed. On the other hand, in a wide-area map, only major (wide) roads, such as national highways, expressways and e.g. prefectural roads, are displayed, while minor roads are omitted, because a wide area occupies the screen.
A situation may occur in which, when the vehicle is travelling on a narrow (minor) road RDS with a detailed map displayed as shown in FIG. 8A, a wide-area map at a scale e.g. one level smaller than that of the detailed map is displayed. In such a case as shown in FIG. 8B, the road RDS on which the vehicle is traveling is not displayed and only a mark CM indicating the vehicle position is displayed, spaced apart from any of the displayed roads.
As described above, if a presently traveled road is not displayed on the display screen when a certain scale is selected, the user (e.g. the driver) does not know the direction in which his vehicle is traveling. Then a need for changing the scale arises. However, changing the scale while driving is dangerous because it distracts the driver. Moreover, it is difficult for the user to determine his vehicle position even by changing the map scales, so that there is a possibility of the user losing his way. The possibility of the user becoming unable to determine the traveling direction is particularly strong if the presently travelled road diverges (branches off).